arXiv subjects

R. Ramazashvili

Publications and source records attributed to R. Ramazashvili.

9 recordsLinked to original sources

arXiv2026

Electric excitation of spin resonance in altermagnetic and antiferromagnetic conductors

We predict electric-dipole spin resonance (EDSR) in altermagnetic conductors: in a magnetic field ${\mathbf H}_\perp$, perpendicular to the magnetization axis, an AC electric field will induce a \textit{spin} resonance peak above the lower threshold frequency $ω_- = 2H_\perp$. In sufficiently clean samples, this peak shall be clearly visible on the background of ohmic absorption. EDSR can thus serve as a diagnostic of altermagnetism in conducting materials and as a means to distinguish it from higher-symmetry antiferromagnetic order.

cond-mat.str-el↗

arXiv2020

Experimental evidence for Zeeman spin-orbit coupling in layered antiferromagnetic conductors

Most of solid-state spin physics arising from spin-orbit coupling, from fundamental phenomena to industrial applications, relies on symmetry-protected degeneracies. So does the Zeeman spin-orbit coupling, expected to manifest itself in a wide range of antiferromagnetic conductors. Yet, experimental proof of this phenomenon has been lacking. Here, we demonstrate that the Néel state of the layered organic superconductor $κ$-(BETS)$_2$FeBr$_4$ shows no spin modulation of the Shubnikov-de Haas oscillations, contrary to its paramagnetic state. This is unambiguous evidence for the spin degeneracy of Landau levels, a direct manifestation of the Zeeman spin-orbit coupling. Likewise, we show that spin modulation is absent in electron-doped Nd$_{1.85}$Ce$_{0.15}$CuO$_4$, which evidences the presence of Néel order in this cuprate superconductor even at optimal doping. Obtained on two very different materials, our results demonstrate the generic character of the Zeeman spin-orbit coupling.

cond-mat.str-el↗

arXiv2013

Magnetic quantum oscillations in the charge-density-wave state of the organic metals $α$-(BEDT-TTF)$_2$MHg(SCN)$_4$ with M = K and Tl

The low-temperature charge-density-wave (CDW) state in the layered organic metals $α$-(BEDT-TTF)$_2$MHg(SCN)$_4$ has been studied by means of the Shubnikov -- de Haas and de Haas -- van Alphen effects. In addition to the dominant alpha-frequency, which is also observed in the normal state, both the magnetoresistance and magnetic torque possess a slowly oscillating component. These slow oscillations provide a firm evidence for the CDW-induced reconstruction of the original cylindrical Fermi surface. The alpha-oscillations of the interlayer magnetoresistance exhibit an anomalous phase inversion in the CDW state, whereas the de Haas -- van Alphen signal maintains the normal phase. We argue that the anomaly may be attributed to the magnetic-breakdown origin of the alpha-oscillations in the CDW state. A theoretical model illustrating the possibility of a phase inversion in the oscillating interlayer conductivity in the presence of a spatially fluctuating magnetic breakdown gap is proposed.

cond-mat.str-el↗

arXiv2003

Magnetotransport near a quantum critical point in a simple metal

We use geometric considerations to study transport properties, such as the conductivity and Hall coefficient, near the onset of a nesting-driven spin density wave in a simple metal. In particular, motivated by recent experiments on vanadium-doped chromium, we study the variation of transport coefficients with the onset of magnetism within a mean-field treatment of a model that contains nearly nested electron and hole Fermi surfaces. We show that most transport coefficients display a leading dependence that is linear in the energy gap. The coefficient of the linear term, though, can be small. In particular, we find that the Hall conductivity $σ_{xy}$ is essentially unchanged, due to electron-hole compensation, as the system goes through the quantum critical point. This conclusion extends a similar observation we made earlier for the case of completely flat Fermi surfaces to the immediate vicinity of the quantum critical point where nesting is present but not perfect.

cond-mat.mtrl-sci↗

arXiv2003

Scaling approach to itinerant quantum critical points

Based on phase space arguments, we develop a simple approach to metallic quantum critical points, designed to study the problem without integrating the fermions out of the partition function. The method is applied to the spin-fermion model of a T=0 ferromagnetic transition. Stability criteria for the conduction and the spin fluids are derived by scaling at the tree level. We conclude that anomalous exponents may be generated for the fermion self-energy and the spin-spin correlation functions below $d=3$, in spite of the spin fluid being above its upper critical dimension.

cond-mat.str-el↗

arXiv2003

Hall Effect in Nested Antiferromagnets Near the Quantum Critical Point

We investigate the behavior of the Hall coefficient in the case of antiferromagnetism driven by Fermi surface nesting, and find that the Hall coefficient should abruptly increase with the onset of magnetism, as recently observed in vanadium doped chromium. This effect is due to the sudden removal of flat portions of the Fermi surface upon magnetic ordering. Within this picture, the Hall coefficient should scale as the square of the residual resistivity divided by the impurity concentration, which is consistent with available data.

cond-mat.str-el↗

arXiv2001

How do Fermi liquids get heavy and die?

We discuss non-Fermi liquid and quantum critical behavior in heavy fermion materials, focussing on the mechanism by which the electron mass appears to diverge at the quantum critical point. We ask whether the basic mechanism for the transformation involves electron diffraction off a quantum critical spin density wave, or whether a break-down in the composite nature of the heavy electron takes place at the quantum critical point. We show that the Hall constant changes continously in the first scenario, but may ``jump'' discontinuously at a quantum critical point where the composite character of the electron quasiparticles changes.

cond-mat.str-el↗

arXiv2000

Onset of antiferromagnetism in heavy fermion metals

There are two views of antiferromagnets. The first proceeds from atomic physics, which predicts that atoms with unpaired electrons develop magnetic moments. In a solid, the coupling between moments on nearby ions then yields antiferromagnetic order at low temperatures. The second, based on the physics of electron fluids or 'Fermi liquids', states that Coulomb interactions can drive the fluid to adopt a more stable configuration by developing a spin density wave. It is presently unknown which view is appropriate at a `quantum critical point', where the antiferromagnetic transition temperature vanishes. Here we describe an atomically local contribution to the magnetic correlations which develops in the metal CeCu$_{6-x}$Au$_{x}$ at the critical gold concentration ($x_c=0.1$) where the magnetic ordering temperature is tuned to zero. This contribution implies that a Fermi-liquid destroying spin-localizing transition, unanticipated for the spin density wave description, coincides with the antiferromagnetic quantum critical point.

cond-mat.str-el↗

arXiv1998

Scaling of magnetic fluctuations near a quantum phase transition

We use inelastic neutron scattering to measure the magnetic fluctuations in a single crystal of the heavy fermion alloy CeCu_5.9Au_0.1 close to the antiferromagnetic quantum critical point. The energy and temperature-dependent spectra obey (E/T) scaling at Q near (1,0,0). The neutron data and earlier bulk susceptibility are consistent with the form 1/X ~ f(Q)+(-iE+bT)^a, with an anomalous exponent a=0.8. We confirm the earlier observation of quasi-low dimensionality and show how both the magnetic fluctuations and the thermodynamics can be understood in terms of a quantum Lifshitz point.

cond-mat.str-el↗